The present invention relates generally to hydraulic positioning and control systems and, more particularly, to positioning and control systems for thrust reverser cowls of turbofan jet engines.
High bypass turbofan jet engines of the type used on large commercial airplanes include a large rotatable turbofan coupled to the forward end of the central shaft of a turbine jet engine. The diameter of the turbofan is considerably greater than that of the jet engine immediately behind it. Rotation of the turbofan by the turbine engine causes the turbofan to produce a fan thrust by blowing air rearwardly around the outer surface of the turbine engine. The fan thrust thus augments the thrust of the turbine engine in powering the airplane. An outer fan housing or cowling encloses the turbofan and channels the fan thrust rearwardly along the outside of the engine.
During landing, and occasionally during taxing of the airplane, it is desirable to reverse the direction of the fan thrust to produce a braking action. Reversal of the fan thrust is accomplished by deployment of a pair of fan thrust reverser cowls which operate to deflect the rearwardly directed fan thrust from the turbofan to a forward direction. The fan thrust reverser cowls form part of the outer fan housing which encloses the turbofan. The reverser cowls are ordinarily semicylindrical in configuration (C-shaped in cross section) such that the two cowls combined form a tubular body sized to match the fan housing. The reverser cowls are normally maintained in a stowed position wherein they form a continuous, faired extension of the trailing edge of the fan housing.
During deployment, the reverser cowls are moved rearwardly of parallel sets of linear hydraulic actuators which connect the reverser cowls to the fan housing. As the reverser cowls are moved rearwardly, an annular gap is opened between the trailing edge of the stationary fan housing and the forward edges of the reverser cowls. The reverser cowls include airflow deflector panels, known as blocker doors, which form part of the inner surfaces of the reverser cowls in their stowed position and which swing inwardly from the inner surfaces as the reverser cowls are moved rearwardly. The blocker doors thus operate in deployment to obstruct the rearward flow of the fan thrust along the annular channel between the reverser cowls and the outer surface of the turbine engine. More specifically, the blocker doors deflect the fan thrust airflow outwardly in generally radial directions through the annular gap between the fan housing and the reverser cowls. The radially deflected airflow is further deflected into a forward direction by sets of forwardly directed airflow vanes which are affixed to the fan housing and are positioned in the gap between the trailing edge of the turbofan housing and the leading edges of the reverser cowls. Thus, the deployment of the thrust reverser cowls causes thrust from the turbofan to be deflected to a forward direction to thereby produce a braking action on the airplane.
Because of certain engine stability requirements and fan blade fatigue stress limitations, it is imperative that the two thrust reverser cowls be moved synchronously when they are being deployed or being retracted. Failure to maintain synchronous movement of the reverser cowls will result in uneven back pressures on the blades of the turbofan. Such uneven back pressures on the spinning turbofan results in high dynamic stresses which can cause damage or even failure of the turbofan.
Previous thrust reverser cowl deployment mechanisms have achieved synchronous movement by use of mechanical interconnections between the two cowls. Specifically, a flexible rotatable shaft has been used to connect the hydraulic actuators that drive the cowls. The flexible shaft was geared to the hydraulic actuators through worm gear assemblies so as to constrain the actuators, and therefore the reverser cowls as well, to move synchronously. This approach met the requirements of synchronous movement, but imposed a substantial weight penalty on the structure due to the weight of the mechanical linkages. Additionally, the mechanical interconnection of the reverser cowls generated installation difficulties, and also interfered with a cowl opening system so as to make it difficult to gain access to the engine for maintenance purposes.
Another problem with mechanically linking the two reverser cowls to achieve synchronous movement is that, in the event of jamming or binding of one cowl during the course of its rearward displacement, the full mechanical load exerted by the actuators of both reverser cowls is applied to the jam point. This creates the possibility of extensive damage to the fan housing and engine structure due to the substantial mechanical forces exerted to deploy the reverser cowls becoming focused on one point of the fan housing structure.
Accordingly, it has been sought to provide a hydraulic positioning system for the thrust reverser cowls that operates to move the cowls independently and yet also ensure synchronous motion of the cowls. As indicated above, independent actuation of the reverser cowls is desirable in order that less powerful actuators may be used to drive each reverser cowl to thereby reduce the maximum potential load on each cowl and thus reduce the possibility of extensive damage in the event of jamming.
Another requirement of a hydraulic positioning system for the reverser cowls is that the full displacement stroke of each cowl be identical even in the event of lagging of one cowl behind the other during deployment. Identical displacement strokes are necessary for the cowls to be properly positioned at the end of each translational movement in deployment or retraction. Although lagging of one cowl behind the other is sought to be avoided for the reasons discussed above, a small amount of lagging may be tolerated and must in fact be anticipated in the design of a suitable positioning system.
This requirement of identical displacement strokes has precluded the use of one conventional approach wherein separate hydraulic actuators drive the cowls and wherein a hydraulic flow divider having a pressure feedback mechanism selectively meters fluid to one hydraulic actuator or the other in the event of a pressure differential between the actuators. With such a system increasing resistance met by one cowl results in greater force being applied to that cowl. However, once a leading cowl becomes fully deployed the flow divider causes the lagging cowl to stall with the result that full deployment of the stalled reverser cowl is not achieved.
Another conventional hydraulic system that has been considered for positioning of the thrust reverse cowls is a servovalve mechanism. Such a mechanism includes a closed loop hydraulic system similar to a primary flight control system. The disadvantage of such a system is its greater complexity due to certain mechanical feedback assemblies which are required, as well as difficulties in rigging such a more complex system in the confined space of the fan housing.
Accordingly, it is an object and purpose of the present invention to provide a hydraulic positioning and control system for fan thrust reverser cowls in a turbofan engine.
It is also an object to provide a hydraulic positioning and control system that provides substantially synchronous translation of the thrust reverser cowls during deployment as well as during retraction.
It is another object of the present invention to provide a hydraulic positioning and control system that achieves the foregoing objects and purposes and which also ensures that the thrust reverser cowls undergo substantially identical full stroke displacements during deployment as well as retraction.
It is yet another object of the present invention to attain the foregoing objects with a hydraulic system that is reliable, simple, contains few moving parts, and which does not impose a significant weight penalty on the engine.
It is a further object of the present invention to provide a hydraulic positioning and control system for a pair of fan thrust reverser cowls whereby each cowl is independently driven to thereby reduce the extent of damage that might result from failure or jamming of a cowl during deployment or retraction.